There is a bug with bukkit build #2815 and latest echopet dev build #20.
When a mob that has effects is spawned with /pet (mobname) it is stuck, doesn't move and is having issues with the effect. Examples are the blaze, enderman and the witch. These mobs have effects, and when spawned, they glitch.

here is a screenshot of the blaze

Fixed: 98d1dc6. Thanks for reporting! 😀